This invention relates to vehicle powertrains having a two-stroke engine configured for combustion with at least 66% excess air and an electric motor in hybrid combination with the two-stroke engine.
Hydrogen is a fuel that can burn reliably in a piston engine with a large amount of excess air. Burning hydrogen, or another fuel with wide flammability limits, with sufficient excess air produces only very small amounts of unburned fuel and very small amounts of oxides of nitrogen as emissions. Slightly lean combustion tends to be ideal for eliminating unburned fuel, and combustion in the presence of 66% or more excess air tends to eliminate oxides of nitrogen when hydrogen is the fuel.
However, prior art vehicle engines and powertrains cannot make very effective and efficient use of hydrogen fuel to take full advantage of extremely lean combustion. A hydrogen-fueled engine must process about twice as much air to effectively eliminate regulated emissions; excess air requires engine displacement be proportionately larger for the same amount of fuel burned and the power produced. Thus, for example, an engine operating with 100% excess air would require twice as much displacement as an engine operating with no excess air to achieve the same power output for the same amount of fuel. Increased size tends to make the engine less efficient, since its friction is relatively greater compared to the power produced. Since hydrogen is relatively expensive and difficult to store, it must be used efficiently.
Two-stroke engines with port scavenging are very simple. They lack separate intake and exhaust strokes and therefore do not keep intake and exhaust gases as well separated as four-stroke engines. The mixing of fresh and burned gases in the scavenging process normally makes the control and reduction treatment of emissions from the two-stroke engine relatively difficult. Without direct injection, escaping intake charge carries fuel to the exhaust, and the exhaust is always lean and cannot be cleaned by conventional catalytic converters. Burned gases remaining in the cylinder also reduce the maximum power that can be produced. As the engine is throttled to low torque and power, more burned gases remain, which can cause poor combustion and additional emissions.
A vehicle powertrain is provided that effectively and efficiently uses hydrogen to take advantage of lean combustion. The powertrain of the invention includes a torque-producing two-stroke engine in hybrid combination with an electric motor and a vehicle transmission. The two-stroke engine includes a cylinder, a piston in the cylinder reciprocally translatable between a top dead center position and a bottom dead center position for a compression stroke and a power stroke, an inlet port for admitting air into the cylinder, and a fuel injector configured to directly inject fuel into the cylinder for combustion. The engine is configured such that the fuel injector injects a quantity of fuel into the cylinder that results in at least 66% excess air in the cylinder during the combustion.
A direct injection port scavenged two-stroke engine configured for very lean combustion tends to eliminate the problems found in prior art lean-burn powertrains and prior art two-stroke engines. Residual gases that cannot be removed by the two-stroke engine are both a contributor to and substitute for excess air that a four-stroke engine must purposefully include to run very lean. A port scavenged two-stroke cylinder might typically contain one-third burned gases (xe2x80x9cretained gasesxe2x80x9d) and two-thirds fresh gases in the midst of the compression stroke. In an engine operating with a large amount of excess air, almost half of the retained gases would actually be air, and all would be useful for limiting oxides of nitrogen. The two-stroke engine would then have an almost two-to-one advantage in power produced per unit of engine displacement over a four-stroke engine, and a port scavenged engine has no conventional valves, so its cost, size, and friction are much better.
The invention also improves upon the prior art by eliminating problems associated with throttling a two-stroke engine. The electric motor provides a substantial amount of peak power for vehicle acceleration, so the two-stroke engine can be configured to operate within a reduced range of power output. The transmission is preferably a continuously variable transmission (CVT) or a closely-stepped ratio transmission. The electric motor may provide some or most of the power for quick response to accelerator pedal xe2x80x9ctip-in,xe2x80x9d and the CVT or closely-stepped-ratio transmission can be overshifted so the engine can be run with little or no throttle margin for acceleration.
The hybrid powertrain also alleviates the effect of lean combustion and excess air on engine size; the motor contributes to power output and thus the engine size may be smaller in hybrid combination with the motor than without a motor. The low cost, mass, and size of the two-stroke engine tends to offset the cost, mass and size of hybrid and CVT or close ratio transmission components. The smoother operation of the two-stroke engine also helps to improve comfort in relatively high torque, low speed overshifted operation compared to the prior art.
The above features and advantages, and other features and advantages, of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawing.